My daughter makes me feel the same way. You're not alone...

It stresses you out because you feel his setting himself up for a giant slap in the face from reality.

Its not up to you nor can you correct his perception of the world and himself. You'll only make it worse. Just be as neutral as you can be. It wont seem like it but he will naturally pick up your lack of genuine enthusiasm and stress signals anyway, but its way more effective it that happens candidly, from my experience.

Also I have two family members like this and... I am like this too. So I've seen it from the inside and the outside. From the outside its clear there are certain obvious things that are being overlooked and certain prerequisites that are obviously not and cannot be met. But the person goes on without the proper rigor of thought. Its like they're just clutching ideas and stringing them together but completely forget about the foundation and the details. They will often do stupid ass calculations out loud or with a calculator which are based on numbers pulled straight out of their asshole. They will assume everything rather than find out, and when they're done laying out their master plan, they'll grab a nice bite to eat, if not already eating, and move straight on to something that has no bearing whatsoever on their goals, like watching TV or playing video games or something. Like, "ok I figured it all out, now let me chillax a bit and allow it to create itself".

From the inside, all you're trying to do is sell the idea! Nothing else. You know what you plan to do and how, and you know exactly what is missing but all you want is to get the other person to listen and to agree, at all costs, so that they can encourage you and you can have just a little more juice to keep working at it in your head. At the same time you using it as a sort of revision exercise. Sometimes when no one is around you will even sit with yourself and talk to yourself as if you're talking to someone and explaining the idea. And when someone shuts you down abruptly or tries to, it hits pretty hard.

And not all of them will be delusional. Most, but at some point there will be a genuine pearl. Again just my opinion from my experience.

It shows that you care. You don't want to see him get expectedly disappointed, so you cringe whenever he mentions these goals seemingly without any plans in place to achieve them. Ultimately you can't control him, so maybe every now and then, hint at concrete steps he could take. It's up to you, what you do, and you could always just try to avoid those conversations entirely if nothing you say is going to change his mind. Best of luck.

thank you everyone, these were super helpful :)

I find my mother incredibly triggering and can't stay there more than 2 hrs. I have to leave. She argues the toss on every subject, no matter what topic I raise she turns it into an opportunity to pontificate as the wisest person in the room. It's gotten worse with the dementia as she doesn't know what she's talking about or why, she's just on a massive roll and is completely switched out and will throw a tantrum if you try to rein her in. I've had a lifetime of her switched out ranting so it does me in a bit.

I have someone at work who is incredibly like her and I can't cope with both of them in my life at one time.

Give me a foolish overenthusiastic dreamer any day. But having said that warn your brother. His enthusiasm in talking to you is what is causing him to lose motivation and is killing the dreams he is trying to bring to life. He has to keep his dreams unspoken if he wants them to come real.

Give him some encouragement. I think he might be anxious, too. And with a low self esteem. Be kinder and help him. He needs your support not your judgement.